The present invention relates to a transmitting apparatus using at least two antenna elements, which executes communication while forming a transmission beam by weighting user signal data (code, rate, frequency, or transmission power) transmitted from each antenna element by an antenna weight and, more particularly, to a transmitting apparatus which has a calibration function of correcting an antenna weight in accordance with the characteristic of a transmission path in the apparatus. The user signal data will simply be referred to as a “user signal” hereinafter.
In, e.g., a cellular mobile communication system, a scheme using adaptive antenna control has been examined aiming at an increase in signal speed/quality and subscriber capacity. According to the adaptive antenna control technique, in an array antenna including three or more antenna elements with very high correlation, a transmission beam pattern is formed by controlling at least one of the phase and amplitude (to be referred to as “phase/amplitude” hereinafter) of a user signal transmitted from each antenna element by using the same frequency band, and directivity is applied in the arrival direction of a desired signal, or null is formed for an interference signal.
In an array antenna transmitting/receiving apparatus having a plurality of transmission paths connected to the respective antenna elements, generally, no ideal transmission directivity pattern can be formed because of the frequency characteristic (phase and amplitude) in constituent elements (including cables) on each transmission path, characteristic variations caused by a temperature or humidity fluctuation, and variations such as a secular change. For this reason, in forming a transmission directivity pattern, variations in phase and amplitude caused by the above-described factors must be compensated for. This operation is called calibration. Calibration includes online calibration and offline calibration. In the former, calibration is executed even during operation of the apparatus, and an updated latest calibration coefficient is applied. In the latter, a calibration coefficient measured before the operation of the apparatus (at the time of installation or shipment) is applied even during the operation.
Conventionally, in the calibration method of the array antenna transmitting/receiving apparatus of this type, a known calibration signal is input to the radio transmitting unit connected to each antenna element, and variations in phase (delay) and amplitude (gain) of each transmission path, which vary every moment independently, are compensated for by using a result obtained by demodulating the calibration signal. The conventional calibration method will be described below in more detail with reference to FIGS. 8 to 11.
FIG. 8 shows an arrangement example of a conventional array antenna transmitting/receiving apparatus which executes online calibration. This array antenna transmitting/receiving apparatus comprises an array antenna 801 including N antenna elements 8021 to 802N, distributor 1 8031 to distributor N 803N, antenna 1 radio transmitting unit 8041 to antenna N radio transmitting unit 804N, antenna 1 signal processing unit 8051 to antenna N signal processing unit 805N, calibration signal generation unit 1 8061 to calibration signal generation unit N 806N, user signal processing unit 807, adder 808, radio receiving unit 809, calibration signal demodulation unit 1 8101 to calibration signal demodulation unit N 810N, and demodulation result processing unit 811.
In the array antenna 801, the N antenna elements 8021 to 802N are arranged closely in an array. They can form a desired transmission directivity pattern by controlling the antenna weight (phase/amplitude) by the user signal processing unit 807. The number N of antenna elements is set to 3 or more to discriminate the antenna from a normal diversity arrangement.
The user signal processing unit 807 outputs user signals weighted by the antenna weight for the respective users to the antenna 1 signal processing unit 8051 to antenna N signal processing unit 805N.
The calibration signal generation unit 1 8061 to calibration signal generation unit N 806N generate calibration signals which are orthogonal to each other in all transmission paths in the baseband and output the signals to the antenna 1 signal processing unit 8051 to antenna N signal processing unit 805N, respectively.
The antenna 1 signal processing unit 8051 to antenna N signal processing unit 805N receive the user signals output from the user signal processing unit 807 and the calibration signals output from the calibration signal generation unit 1 8061 to calibration signal generation unit N 806N, execute spread modulation and multiplexing, and output the resultant multiplexed signals to the antenna 1 radio transmitting unit 8041 to antenna N radio transmitting unit 804N, respectively. The user signal and calibration signal code-multiplexed by each antenna signal processing unit are orthogonal to each other.
The antenna 1 radio transmitting unit 8041 to antenna N radio transmitting unit 804N receive the multiplexed signals output from the antenna 1 signal processing unit 8051 to antenna N signal processing unit 805N, execute digital/analog conversion, quadrature modulation, frequency conversion from the baseband to radio frequency band, amplification, frequency band limitation, and the like for the multiplexed signals in the baseband, and output the signals to the distributor 1 8031 to distributor N 803N, respectively.
The distributor 1 8031 to distributor N 803N receive the multiplexed signals output from the antenna 1 radio transmitting unit 8041 to antenna N radio transmitting unit 804N and output the multiplexed signals to the antenna elements 8021 to 802N, respectively. The distributor 1 8031 to distributor N 803N also partially distribute the powers to the adder 808.
The adder 808 receives the multiplexed signals output from the distributor 1 8031 to distributor N 803N, combines the multiplexed signals in the radio frequency band, and outputs the signal to the radio receiving unit 809.
The radio receiving unit 809 receives the multiplexed signal output from the adder 808, executes frequency band limitation, amplification, frequency conversion from the radio frequency band to baseband, quadrature demodulation, analog/digital conversion, and the like, and outputs the signal to the calibration signal demodulation unit 1 8101 to calibration signal demodulation unit N 810N.
The calibration signal demodulation unit 1 8101 to calibration signal demodulation unit N 810N receive the multiplexed signal output from the radio receiving unit 809 and extract, from the multiplexed signal, the calibration signals in the transmission paths 1 to N. More specifically, the calibration signal demodulation unit 1 8101 extracts the calibration signal generated by the calibration signal generation unit 1 8061. Similarly, the calibration signal demodulation unit 2 8102 to calibration signal demodulation unit N 810N extract the calibration signals generated by the calibration signal generation unit 2 8062 to calibration signal generation unit N 806N, respectively. As described above, the calibration signals are orthogonal to each other. Hence, they can be extracted by despreading the multiplexed signal.
The calibration signal demodulation unit 1 8101 to calibration signal demodulation unit N 810N detect transmission path 1 demodulation symbol point (phase/amplitude information) to transmission path N demodulation symbol point (phase/amplitude information) from the extracted calibration signals. The transmission path 1 demodulation symbol point is added the phase/amplitude variation of the transmission path including the antenna 1 radio transmitting unit 8041. This also applies to the transmission path 2 demodulation symbol point to transmission path N demodulation symbol point. The detected transmission path 1 demodulation symbol point to transmission path N demodulation symbol point are output to the demodulation result processing unit 811.
The demodulation result processing unit 811 receives the transmission path 1 demodulation symbol point to transmission path N demodulation symbol point output from the calibration signal demodulation unit 1 8101 to calibration signal demodulation unit N 810N, calculates calibration coefficients as the correction information of the respective transmission paths from these demodulation symbol points, and outputs the calibration coefficients to the user signal processing unit 807. More specifically, the calibration coefficients are calculated in the following way.
It is defined that the transmission path including the antenna 1 radio transmitting unit 8041 is always the reference route. The calibration signal generated by the calibration signal generation unit 1 8061 is demodulated by the calibration signal demodulation unit 1 8101. The obtained symbol point is defined as a reference symbol point S1, as shown in FIG. 9. The symbol point generated by the calibration signal generation unit 2 8062 and demodulated by the calibration signal demodulation unit 2 8102 is defined as S2. The symbol point generated by the calibration signal generation unit N 806N and demodulated by the calibration signal demodulation unit N 810N is defined as Sn. The demodulation result processing unit 811 detects a phase difference θ2 and amplitude ratio r2=B/A between S1 and S2 and a phase difference θn and amplitude ratio rn=C/A between S1 and Sn. When normalization with respect to the reference symbol point S1 is done, the symbol points can be expressed, as shown in FIG. 10. At this time, r2 to B/A=B′/1, and rn=C/A=C′/1 although the values of the phase differences θ2 and θn and the amplitude ratios r2 and rn do not change. The demodulation result processing unit 811 outputs the values θ2 to θn and r2 to rn to the user signal processing unit 807 as calibration coefficients for each calibration period.
The user signal processing unit 807 corrects the user signals, which have undergone antenna weight (phase/amplitude) control for the respective users, by using the calibration coefficients output from the demodulation result processing unit 811.
Even when the phase/amplitude varies in the transmission paths during the operation of the apparatus, this array antenna transmitting/receiving apparatus can correct the generated phase/amplitude variation by giving the calibration coefficients to the user signal processing unit 807. When the user signals which form a transmission directivity pattern by antenna weight (phase/amplitude) control are transmitted after correction of the phase/amplitude variation in the transmission paths, an accurate transmission directivity pattern can be formed (e.g., Japanese Patent Laid-Open No. 10-336149).
FIG. 11 shows an arrangement example of a conventional array antenna transmitting/receiving apparatus which executes offline calibration. This array antenna transmitting/receiving apparatus comprises an array antenna 1101 including N antenna elements 11021 to 1102N, antenna 1 radio transmitting unit 11041 to antenna N radio transmitting unit 1104N, antenna 1 signal processing unit 11051 to antenna N signal processing unit 1105N, calibration signal generation unit 1 11061 to calibration signal generation unit N 1106N, user signal processing unit 1107, adder 1108, radio receiving unit 1109, calibration signal demodulation unit 1 11101 to calibration signal demodulation unit N 1110N, and demodulation result processing unit 1111.
When the calibration coefficients in the apparatus are to be measured at the time of installation or shipment of the apparatus, the arrangement from the adder 1108 is connected to the antenna 1 radio transmitting unit 11041 to antenna N radio transmitting unit 1104N. In operation of the apparatus, the arrangement is disconnected from the antenna 1 radio transmitting unit 11041 to antenna N radio transmitting unit 1104N, and instead, the array antenna 1101 is connected.
The calibration coefficients measured at the time of installation of the apparatus are stored in the user signal processing unit 1107. An effect to transmission directivity pattern formation can be obtained by adding the calibration coefficient to the antenna weight even during the operation. This calibration method is effective especially for an apparatus which shares the local signal or operation clock to be supplied to all antenna radio transmitting units because the influence of the local signal as the dominant factor of the phase variation is invisible.
However, in the conventional array antenna transmitting/receiving apparatus shown in FIG. 8, calibration is always executed during operation. Hence, the calibration signal as an unnecessary radio wave for the user signal is continuously transmitted. In addition, the spreading devices equal in number to the antenna elements always operate. This suppresses the remaining system capacity of carrier power in each transmission path.
In this array antenna transmitting/receiving apparatus, the calibration signals transmitted from all the antenna radio transmitting units at the same time are demodulated at a short period to calculate the calibration coefficients. Since this requires high-speed arithmetic processing, the load of signal processing is large.
In this array antenna transmitting/receiving apparatus, the calibration signal with the same power is always multiplexed and transmitted with respect to the user signal whose set power increases or decreases. As a result, the calibration signal looks like interference for the user signal more than necessary, and the calibration accuracy is low.
In the conventional array antenna transmitting/receiving apparatus shown in FIG. 11, the calibration coefficient measured at the time of installation or shipment is used even in executing calibration during the operation. For this reason, the information of total transmission power information as one of the major factors of the phase/amplitude variation in the transmission path is not fed back. Hence, correction cannot be done in consideration of the saturation characteristic in the constituent element in the transmission path. As a result, accurate calibration is impossible.
Problems of calibration of an array antenna transmitting/receiving apparatus using adaptive antenna control have been described above. However, even calibration of a transmitting apparatus which executes transmission diversity has the same problems. Transmission diversity is a technique of increasing the terminal reception quality by executing transmission from two antenna elements of a radio base station transmitting apparatus, which are arranged almost without correlation, by using the same frequency band and controlling the phase/amplitude of the user signal transmitted from each antenna element.